Matter waves: Be fruitful and multiply

At the heart of an optical laser, a few photons flying in formation trigger an avalanche of other photons that join their ranks. The new recruits spring from atoms shedding energy. That amplification yields a laser beam in which hordes of these elementary particles of light, equivalent to electromagnetic waves, all line up, crest to crest and trough to trough. This coordination is known as phase coherence.

Left: Atom cloud, or seed (top), comes from condensate (bottom). Right: After being inside an excited condensate, seed has recruited atoms from it. MIT Research Lab. of Electronics

Now, U.S. and Japanese researchers have demonstrated coherent amplification of atoms much like the photon cascade in lasers.

According to the tenets of quantum mechanics, atoms, like photons, behave both as particles and as waves. However, atoms can’t typically spring from energy. Atom, or matter-wave, amplification therefore involves transferring atoms from reservoirs of other atoms.

Physicists have been eager for a demonstration of matter-wave amplification because it provides a striking confirmation of quantum mechanical predictions. “The fact that they can do it and get clear evidence for it is very impressive. These are wonderful experiments,” comments Charles W. Clark of the National Institute of Standards and Technology (NIST) in Gaithersburg, Md.

In earlier experiments, “people have seen an amplification process in an indirect way, but nobody has built an amplifier” before, says Wolfgang Ketterle of the Massachusetts Institute of Technology. His team reports in the Dec. 9, 1999 Nature that it has amplified sodium atoms.

Since 1997, physicists have made atoms emerge coherently in drips or beams from gases of ultracold, coherent matter known as Bose-Einstein condensates (SN: 7/15/95, p. 36). These emissions are called atom lasers (SN: 2/1/97, p. 71).

In the new experiment, the team used optical lasers to generate coherent “seeds” of sodium atoms within regions of frigid, cigar-shaped condensates. The experimenters manipulated the seeds to have quantum states that were different from those of the rest of the condensate atoms, and therefore, move with different speeds and directions than those of the surrounding cloud.

By then pumping the condensates with optical lasers, the scientists spurred many condensate atoms to emit photons with a precise energy and direction. Atoms that lost a certain amount of energy became part of the seed, making it grow larger.

Ketterle’s team observed 10-to-100-fold leaps in numbers of atoms in the cloud that began as the seed. The increase depended on pump-laser intensity. The amplification is not perfect, Ketterle notes. It appears to distort the original coherent state of the seed into a slightly different form.

In the Dec. 17, 1999 Science, researchers from the University of Tokyo and Gakushuin University in Tokyo with collaborators from NIST, not including Clark, revealed that they had used a similar technique to amplify rubidium atoms. They reported at least a 10-fold gain in atoms.

The achievement of matter-wave amplification suggests that atom optics might one day prove as rich and rewarding as recent developments using conventional lasers that beam photons.

The new atom amplifiers join the growing kit of atom-optics tools (SN: 5/8/99, p. 296) that are beginning to find use in improved devices for measuring rotation, gravity, and time and for creating minute structures with beams of atoms.

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